that's the law. you can't tell by looking at someone if a judge has declared a person to be mentally defective. you cannot tell by looking at a person if they are a convicted felon. if you are either of those things, the law says you cannot buy a gun. here is how we check to see if you are a felon or mentally ill before you buy a gun in this country. >> the crew went north to kings port, tennessee to a saturday morning local gun show held in a convention center. it was a smith & wesson mp 45 caliber semi-automatic that first caught our producers eye. asking price, $625. >> it's not brand spanking new. >> cash and carry? >> but it's early and the team opts to keep looking. 10 to 20 minutes later, they circle back to the same table, negotiating for the same gun. >> it's a deal. no background check. it's not needed for a private sale. but the serial is legally required to check id like a driver's license, to make sure the buyer is not from out-of-state. in this case, no identification asked for, no pape